The tightening torque of conventional bolt or nut tightening devices is adjusted and recognized by referring to a torque adjusting dial (indicated at 10 in FIG. 24) provided on the main body of the tightening device and manually tightening the bolt or nut additionally with a torque wrench.
Stated more specifically, the torque adjusting dial 10 is first set at a value slightly lower than the desired tightening torque value, and a bolt or nut (hereinafter referred to typically as “nut”) is tightened.
Thus, a torque value serving as a rough target value is set on the dial 10 utilizing the fact that the tightening torque of the tightening device shown in FIG. 22, Graph X and the load current of the tightening device shown in Graph Y are in a nearly proportional relationship. Upon the load current reaching a target value, the motor of the tightening device ceases rotating. The actual torque at this time differs from the value set on the adjusting dial, for example, owing to the gear efficiency of the reduction mechanism incorporated in the main body of the tightening device.
Accordingly, the worker additionally tightens up the nut with a torque wrench equipped with a torque indicator to recognize the actual tightening torque.
The procedure including the manipulation of the torque adjusting dial 10, tightening of the nut by the tightening device and additional tightening of the nut with the torque wrench is repeated a number of times in order to set the dial 10 so that the actual tightening torque of the tightening device becomes the desired torque value.
The nuts used for bridges and like large steel-frame structures are large-sized, and manually using the torque wrench for tightening heavily burdens the worker. Using the torque wrench also poses many safety problems in view of the work environment where the tool is not always easily usable and which is likely to involve, for example, an elevated site and unstable scaffolding.
Accordingly, a tightening torque measuring implement 9 for use as attached to the tightening device main body 1 has been proposed as shown in FIG. 24.
The instrument utilizes the fact that the tightening torque shown in FIG. 22, Graph X and the amount of strain of the tightening device shown in Graph Z are in a generally proportional relationship.
The tightening device main body 1 has a first output shaft 12 and a second output shaft 13 which are rotatable in opposite directions to each other and are coaxially provided. The usual tightening work is conducted by attaching a tightening socket 21 to the first output shaft 12 and a reaction force receiver 22 to the second output shaft 13, engaging the socket 21 with a nut N, and positioning the reaction force receiver 22 in bearing contact with another nut or like projection (not shown) in the vicinity of the nut N.
The tightening torque measuring instrument 9 is used as connected between the first output shaft 12 of the device main body 1 and the socket 21. The instrument 9 comprises a solid shaft portion 91 provided at its base end with a square cavity 92 for a square shaft portion 12a of the first output shaft 12 of the device main body 1 to fit in, and has at its forward end a square rod 93 fittable into the base end of the tightening socket 21.
A strain gauge 47 is affixed to the surface of the solid shaft portion 91. The shaft portion 91 is provided therearound with a circuit board, torque indicator and battery (none shown).
The adjustment and recognition of torque of the tightening device having the instrument 9 attached thereto is done in the same manner as described above. The torque adjusting dial 10 on the device main body 1 is set at a value slightly lower than the desired torque value.
The reaction force receiver 22 is attached to the second output shaft 13 of the device main body 1, the tightening socket 21 is engaged with a nut N, the reaction force receiver 21 is cause to bear against a projection in the vicinity of the nut N, and the nut N is tightened.
The motor comes to a halt upon the value of current through the motor of the tightening device reaching a specified value. The indicator shows a tightening torque value corresponding to the amount of strain of the shaft portion 91 of the instrument 9 at this time.
The manipulation of the torque adjusting dial 10 and the tightening of the nut are repeated several times, and the dial 10 is set so that the actual tightening torque value on the indicator of the tightening device becomes the desired torque value.
Since the desired torque value can be set by manipulating the torque adjusting dial with reference to the actual tightening toque value on the indicator, there is no need for the worker to additionally tighten the nut with the torque wrench to measure the tightening torque.
This obviates all the problems as to the burden of labor for additionally tightening the nut with the torque wrench, hazard, etc.
The tightening torque measuring instrument 9 described is of the single shaft type, and there is a need to attach the reaction force receiver 22 to the second shaft 13 of the tightening device main body 1.
In the case where the tightening socket 21 is to be attached directly to the first output shaft 12 of the tightening device main body 1, a reaction force receiving arm 20 of the reaction receiver 22 is to be elongated in a direction along the axis of the tightening socket 21 by an amount corresponding to the length of the socket 21.
However, if the tightening torque measuring instrument 9 is interposed between the device main body 1 and the socket 21, the reaction receiving arm 20a needs to be further lengthened by an amount corresponding to the length of the instrument. This increases the distance between the second output shaft 13 of the device main body 1 on which the tightening reaction acts and the remote end of the reaction force receiving arm 20a in bearing contact with a counter member which is to be actually subjected to the tightening reaction force. In this case, the reaction force acting on the arm 20a exerts a great force in a direction to knock the arm down. Accordingly, the device main body 1, the torque measuring instrument 9 and the socket 21 to be in alignment with the axis of the nut when tightening the nut will fail to retain their axes in alignment with stability, possibly permitting the indicator to show an inaccurate torque value due to an inclination relative to the axis of the nut.
After the completion of adjustment of toque of the tightening device main body, it is usual practice to remove the instrument 9 and attach the socket 21 directly to the main body 1 for a tightening operation. It is then necessary to replace the arm of the reaction force receiver 22 by the shorter arm. Furthermore, the difference in length between the arms results in a difference in torque transmission efficiency. More specifically, the tightening torque value differs when the torque measuring instrument 9 is attached to the device main body and when the instrument is removed.
An object of the present invention is to provide a torque measuring unit and a torque indicating tightening device which are minimized in the difference in tightening torque value when the torque measuring unit is attached to the device main body and when the unit is removed.